U.S. Pat. No. 3,540,793 (Araujo et al.) discloses that photochromic glasses, based on silver halide crystals, can be rendered polarizing in the darkened state. This dual effect is accomplished by thermally softening and stretching the softened glass. This elongates the silver halide crystals and generates a form birefringent effect in the glass. When the glass is darkened, the anisotropy of the silver halide particles is manifested in an observed dichroism.
Subsequently, U.S. Pat. No. 4,304,584 (Borrelli et al.) reports that a permanent polarizing effect can be obtained. This is accomplished by heat treating the stretched glass in a reducing atmosphere to chemically reduce the elongated silver halide crystals to elongated particles of metallic silver.
Still later, it was learned that the polarizing effect could be extended to the infra-red portion of the spectrum. This is reported in U.S. Pat. No. 4,479,819 (Borrelli et al.).
Glass polarizers, produced from silver halide-containing glasses as just described, have enjoyed commercial success in applications employing radiation in the infra-red portion of the spectrum. However, the parallel and perpendicular absorption curves reverse in magnitude and cross over in the visible wavelength region at about 480 nm. This has prevented the silver halide glasses from being considered for use as a visible polarizer, that is, for applications that require polarization in the same direction across the entire visible wavelength region of the spectrum. As used in this application, the term "visible polarizer" means a polarizer that provides effective polarization of radiation across the entire visible wavelength region of the spectrum. This is essentially the wavelength range of 400-700 nm.
U.S. Pat. No. 3,325,299 (Araujo) discloses phototropic (now known as photochromic) glasses in which copper and/or cadmium halide crystals are precipitated to impart photochromic behavior. The disclosed glasses have R.sub.2 O--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SiO.sub.2 base compositions. These oxides total at least 85% of the glass composition. Copper and/or cadmium oxides, together with sufficient chlorine, bromine and/or iodine to react stoichiometrically with at least 0.3% copper and/or cadmium, are added to the base glass composition.
U.S. Pat. No. 5,281,562 (Araujo et al.) is directed to a family of glasses which also contain a cuprous and/or cadmium halide crystal phase, and which are strongly absorbing in the ultraviolet region of the radiation spectrum. They have a sharp spectral cut-off of shorter wavelengths at about 400 nm. However, they are non-photochromic, and may be essentially colorless.
The application explains that cadmium is not required to secure absorption of ultraviolet radiation, and imparts no exceptional benefit to the glass. The halide crystals are effective to absorb ultraviolet radiation, but cadmium is known to be extremely toxic. Accordingly, its presence is severely restricted, and it is preferably completely avoided. Therefore, while cadmium halide glasses are within the scope of the present invention, reference throughout will be to copper halide glasses.
It has been found that the copper and cadmium halide photochromic glasses disclosed in U.S. Pat. No. 3,325,299 can also be rendered polarizing in the darkened state. The glass is thermally softened and stretched, or otherwise elongated. This action elongates the halide crystals and is described in detail in U.S. Pat. No. 3,954,485 (Seward III, et al.).
Japanese Published Application S.N. 4-279,337 discloses glasses that may be a silicate, a borate, or a borosilicate, and that may have copper halide crystals formed within the glass. The application further discloses that such glasses may be rendered polarizing in the infra-red for telecommunication purposes. This is accomplished in the same manner as in the silver halide crystal-containing glasses noted above. That is, the glass is stretched to elongate the crystals, and fired in hydrogen to at least partially reduce the copper halide to metallic copper.
Heretofore, polarizers useful in the visible wavelength region of the spectrum, that is, 400-700 nm, have been produced from plastic materials. The disadvantages of such materials are well known. They cannot withstand elevated temperatures, scratch easily, bleach under intense light, and may undergo distortion under pressure.
The present invention provides a glass polarizer that is effective throughout the entire visible wavelength portion of the spectrum. The glass polarizer has a sharp cutoff in transmission at about 400 nm, which removes harmful ultraviolet rays. This is of particular value in LCD displays to protect the color filter. The invention further provides a method of producing such a glass polarizer from glasses containing copper halide crystals.